eFinder assembly with lens holder, FR237 focal reducer and extension tube, for ST-402ME (and ST5C, ST-237/A, STV)
The SBIG FR237 Focal Reducer and eFinder Assembly is actually several accessories in one. First and foremost, it is a variable focal reducer that will reduce an f/10 optical system to either f/5.95 or f/3.75 for shorter exposure, wider field CCD imaging. In addition, it is an ultra-compact wide field telescope that may be used for imaging, as an electronic finder, or for autoguiding.
Three parts make up the assembly. The first is an achromatic focal reducer lens. Second is a long eFinder extension tube. Third is a short 1.25" diameter extension tube. Both the eFinder tube and the short extension tube have multiple knife edge baffles machined into their interiors to improve image contrast.
The focal reducer lens is mounted in a 1.25" cell. It threads into the 1.25" nosepiece of SBIG STV, ST-237, ST-5C, and ST-4 CCD cameras (or any other CCD camera with a relatively small detector chip and a 1.25" nosepiece) for operation at f/5.95. When you install the short extension tube between the reducer lens and the camera, the focal ratio is further cut to f/3.75.
The ST-5C, for instance, uses a relatively small 3.2 by 2.4mm CCD detector that makes it inconvenient to image extended objects with longer focal length telescopes. For example, its field of view with an 8" f/10 telescope is only 4.1 by 5.5 arc minutes. The FR237 focal reducer lens plus short extension tube reduce the f/10 focal ratio to f/3.75. At this shorter focal ratio the image area increases to 11 by 14.7 arc minutes. That’s over seven times the area, with a 7x increase in sensitivity to extended and nebular objects. Since the pixel size is still smaller than a star’s image under typical seeing conditions, no resolution is lost. The image improvement is dramatic with diffuse nebulas and galactic objects.
Although the STV and ST-237 use a detector that is twice the size of the detector used in the ST-5C, the same increase in field of view and sensitivity apply. The ST-237 has a 6 by 8 arc minute field with an 8" f/10 scope. With the FR237 operating at f/3.75, the field of view increases seven times in area to 16 by 21 arc minutes and extended objects appear 7 times as bright. This can be particularly important with the STV video camera where you generally prefer to keep exposure times as short as possible.
The eFinder function of the FR237 is recommended for the STV, but it works equally well with the ST-237 and ST-5C cameras. Only the focal reducer lens is used on the ST-4, however, because the eFinder extension tube attached to the camera head by t-threads and the 1.25" nosepiece on the ST-4 does not unscrew.
To use the eFinder assembly on the STV, ST-237 or ST-5C, simply unscrew the camera’s 1.25" nosepiece and replace it with the eFinder tube. The focal reducer lens is inserted into the front of the eFinder extension tube and held in place by two nylon thumbscrews to form a 100mm focal length f/4 telescope. When mounted piggyback on a larger scope, the combination forms an electronic finderscope and guide scope with a 2.7 by 2.0 degree field of view (1.4 by 2.0 degrees with the ST-5C). This image scale makes comparison of the displayed image with a star chart very easy. This combination is also sensitive enough to show any star in a Uranometria 2000.0 star chart with only a 3 second exposure.
As a guide scope, the eFinder will autoguide to better than 1 arc second accuracy (depending on the quality of your mount). With its wide field of view there will virtually always be a guide star in the field of view without having to search for one. This means that the eFinder assembly can be securely fastened without being concerned about having to adjust its pointing off-axis to find guide stars. By locking the eFinder down tight on the main scope you minimize the problems associated with differential flexure of a guide scope. The CCD head and eFinder assembly is only about 5" long overall and weighs little more than a pound. This makes it exceptionally easy to guide telescopes up to about approximately 100" in focal length (a 10" f/10, for example). Of course, no separate guide scope can detect a shift in the position of an SCT’s primary mirror. For this reason, or when doing longer exposures with long focal length scopes, the STV equipped with the FR237 may be used in an off-axis guider for best results.
|Manufacturer||Santa Barbara Instrument Group (SBIG)|